The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Compression-ignition internal combustion engines operate at lean air/fuel ratios to achieve desirable fuel efficiencies. Lean engine operation may produce oxides of nitrogen (NOx) when nitrogen and oxygen molecules present in engine intake air disassociate in the high temperatures of combustion. Rates of NOx production follow known relationships in the combustion process, for example, with higher rates of NOx production being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. NOx molecules may be reduced to nitrogen and water in aftertreatment devices. Efficacy of known aftertreatment devices is dependent upon operating conditions including operating temperature, which is associated with exhaust gas flow temperatures and engine air/fuel ratio. Aftertreatment devices include materials prone to damage or degradation when exposed to elevated temperatures and/or contaminants in the exhaust gas feedstream.
Aftertreatment systems purify exhaust gases by filtering, oxidizing and/or reducing constituents in an exhaust gas feedstream. Three-way catalytic devices (TWC) oxidize and reduce exhaust gas constituents. NOx adsorbers store NOx, which may be subsequently desorbed and reduced under specific engine operating conditions. Diesel particulate filters (DPF) are able to remove particulate matter in the exhaust gas feedstream through mechanical filtering. A selective catalytic reduction device (SCR) includes catalytic material that promotes the reaction of NOx with a reductant such as ammonia (NH3) to produce nitrogen and water. Reductants, e.g., urea, may be injected into an exhaust gas feedstream upstream of the SCR device. Reductants, e.g., NH3, may also be generated in an exhaust gas feedstream upstream of the SCR device during specific engine operating conditions. Catalytic materials used in SCR devices have included vanadium (V) and tungsten (W) on titanium (Ti) and base metals including iron (Fe) or copper (Cu) with a zeolite washcoat.